1. Field of the Invention
The present invention relates to a dot-matrix printer wherein characters are printed in the form of dots, and more particularly to a dot-matrix printer capable of effecting an overlap printing wherein same characters are at least partially superimposed on each other.
2. Discussion of the Prior Art
In a dot-matrix printer, characters such as alphabetic letters, Chinese characters, Japanese "kana" letters, numerals and symbols are printed by forming dots on a recording medium, in a dot-matrix pattern, such that columns of dots each of which consists of a suitable number of dots and extends in a given direction are arranged at a predetermined printing pitch (hereinafter referred to as "nominal printing pitch") in a printing direction which intersects the direction of extension of the columns of dots. Generally, the dot-matrix printer includes (a) a print head having a plurality of dot-forming elements each for forming a dot such that the dots printed by the dot-forming elements are arranged in a column extending in the above-indicated given direction, (b) feeding means for moving the print head in the above-indicated printing direction, and (c) printing control means for controlling the dot-forming elements to be selectively activated to print the corresponding dots, according to dot information representative of ordinary characters to be printed, while the print head is moved by the feeding means.
Some dot-matrix printers are capable of effecting an "overlap" printing wherein two printings are partially superimposed on each other, with a suitable offset distance therebetween in a certain direction, to print a bold-faced character, or the two printings are completely superimposed or overstruck on each other, to print a character with increased darkness or density. In these known dot-matrix printers, the print head is first fed in a predetermined printing direction to effect a normal first printing of characters, and is then returned in the opposite direction to the beginning of the printed line of characters, so that a second printing operation is effected also in the same printing direction. If the printing positions (column positions of dot-matrix pattern) are the same in the first and second printing operations, the characters are printed with higher density. If the printing positions of the second printing are offset in a suitable direction from those of the first printing, the characters are bold-faced or thickened.
In the known dot-matrix printers, however, the overlap printing requires two printing movements of the print head, one for the normal printing by one group of dot-forming elements, and the other for the offset printing by the same group of dot-forming elements. Consequently, the known printers suffer from a comparatively long printing time for the overlap printing, and also suffers from relatively large error in the relative position between the normal first printing and the second offset printing of the characters, due to positioning error of the print head in the first and second movements for the overlap printing, resulting in insufficient print quality in the overlap printing.